Ethylene recovery system



HsloH w. WANG ETAL 3,187,064

June 1, 1965 AETHY'LENE REGOVERY SYSTEM 5 Sheets-Sheet l Filed May 9,1962 y June 1, 1965 Filed may 9, 1962 HSIOH W. WANG ETAL v ETHYLEN-REQOVERY SYSTEM 3 Sheets-Sheet 2 .SPL/ TTEJ? BOTTOMS GAS F550 mvEN'roRsv5/0H M WAN@ CHA l Ml. CHEN WILL/AM C. CHA TMA/v ATTORNEY June 1,1965SICH w. WANG Em 3;18v,04

ETHYLENE RECOVERY SYSTEM Filed May 9, 1962 3 SheetS-Shee 3 INVENTGRSfis/0H uf. WANG CHAI M .CHEN

u//LL/AM c: amr/MN l BY Mal/m Amm United States Patent O w ETHYLENERECVERY SYSTEM Hsieh W. Wang and Chai Wei Chen, Freeport, NX., andWilliam C. (hatman, Matawan, Nd., assignors to Foster WheelerCorporation, New York, N.Y. a corporation ot New York I Filed May 9,1.962, Ser. No. ESA-i3 lill Claims. (Cl. Zoll-577) This inventionrelates to a method and apparatus for treating a mixture of light gases,including ethane and lighter hydrocarbons together with hydrogen, andsmall quantities of C3 and heavier hydrocarbons, for the recovery ofethylene.

More particularly, the invention relates to improvements in a processfor the recovery and purilication of ethylene from a mixture of normallygaseous components, which process includes the steps of cooling andcompressing the mixture and subjecting it to low temperaturefractionation under high pressure for the removal of methane andhydrogen, and the step of hydrogenating acetylene in the mixture toconvert it to ethylene.

The invention is of particular use in obtaining from a hydrocarbon feedmixture containing ethane and heavier and lighter hydrocarbons, and inaddition absorber lean and/or sponge oil and small amounts of methanoladded prior to or during refrigeration, an ethylene product gas which issubstantially pure (approximately 99.9% plus) and free of trace amountsof acetylene. Typical suitable gases which contain ethylene are refinerygases, and gases produced by pyrolysis of saturated hydrocarbons.

In the recovery of ethylene from a mixture of light gases, includingmethane, hydrogen, acetylene, and ethane, a process of the type to whichthe-invention relates may include steps involving completelydeethauizing the mixture in an absorber-deethanizer, and passing theabsorber overheadgases containing ethane and lighter hydrocarbons andhydrogen, together with a small quantity of C3 hydrocarbons and absorberlean and/or sponge oil, to hydrogenation and caustic treating facilitiesin order to remove sulphur compounds and a major portion of theacetylene present.

Subsequently, the mixture may be compressed, dried, subjected torefrigeration, and passed to a demethanizer to remove methane andhydrogen from the mixture. After that, the demethanizer bottoms arevaporized and fractionated in ethylene-ethane splitter to recover anethylene fraction as an overhead or sidestream product from thesplitter.

Recent manufacturing specifications for the recovery of ethylene requirethat all but about five ppm. of acetylene be removed from the ethylene.In order to obtain such a high degree of purity, it is necessary to usea two-stage hydrogenation process, with the second stage followingdemethanization, and although a nickel catalyst may be used in the rststage, a cobalt molybdate catalyst is required for the second stage.

1n this sequence of operations, two problems have been encountered. Theiirst is that prior to demethanization, which is at a low temperature,methanol is periodically or continually injected into the gaseous feedstream in order to prevent freezing.

It has been found that the methanol in the system acts as a poison tothis second stage hydrogenation catalyst. ln particular, it wasdiscovered that a feed gas into the hydrogenation second stage,containing about 1,000 ppm. methanol (CH3OH), caused increased acetyleneleakage after 26 hours running time.

A second problem occurring in the above sequence of operations is thatlean and/or sponge oil is carried with the gaseous phase and ultimatelyleaves the system with the ethylene-ethane splitter bottoms material.This oil ldldd Patented ,lune l, i955 fr@ Li substantially raises thebottoms temperature in the splitte but in order to achieve good heatrecovery, the bottoms temperature ideally should be maintained as low aspossible.

rthese disadvantages are overcome in accordance with the presentinvention, by subjecting the vaporized dernethanizer bottoms tocounter-current contact in a minimum tray scrubber with a scrubbingliquid capable of removing methanol and heavier hydrocarbons, forinstance, the lean or sponge oil from the stream. The scrubbing liquidused is a (I2-rich liquid withdrawn from the ethane-ethylene splitter,and may be withdrawn from any tray of the splitter. ln counter-currentcontact with the gaseous stream, the scrubbing liquid condenses themethanol and heavier hydrocarbons.

ln an embodiment of the invention, the reboiler of the demethanizationis associated with the scrubber column in a novel manner which avoidsthe use of a second heat exchanger for vaporizing the demethanizerbottoms in the scrubber.

ln a third embodiment, a main portion of the demetbanizer bottomsmaterial is withdrawn from a lower part of the demethanizer tower as aside stream Vapor. The vapor side stream contains essentially all of theethylene and ethane present in the demethanizer, whereas a small bottomsliquid stream also Withdrawn from the demethanizer contains the bulk ofthe methanol andk heavier hydrocarbons. T he vapor side stream is passedthrough `the scrubber column to remove the remaining portion of themethanol and heavier hydrocarbons withdrawn with it, but as in thesecond embodiment described above, the arrangement avoids the use of asecond heat exchriger.

The invention and the advantages thereof will become more apparent uponconsideration of the following speciiication and detailed ydescriptionwith reference to the accompanying drawings, in which:

Fl-GURE l represents a schematic iiow diagram of an ethylene recoveryprocess embodying the invention;

FGURES 2 and 3 are schematic flow diagrams of different arrangements ofan embodiment of the invention; and

FGURE 4 is a schematic flow diagram of a third embodiment of theinvention.

Referring to FlGURE l, a gaseous product containing ethylene, and lowerand higher boiling gaseous compounds including methane and hydrogen,acetylene, ethane, and other hydrocarbons having a higher boiling pointthan ethylene, obtained from a pyrolysis furnace or a thermal crackingunit, is quenched in an oil quench vessel l2 and passed by a line lethrough a compressor i6, heat exchanger ll and a separator drum Ztlwhere it is compressed to a superatmospheric pressure and cooled to adesired temperature. The cooled gaseous stream is then fed underpressure in line 22 to an absorber-deethanizer 24 Where it is subjectedto counter-current contact with a lean and/or sponge oil admittedthrough line 2li under reiiurr conditions such that a C2 and lowerboiling gaseous fraction is separated from a higher boiling fraction,the latter containing C3 and heavier hydrocarbons such as propane,propylene and butane.

The gaseous fraction withdrawn in line 23 and consisting largely ofethane and lighter hydrocarbons and hydrogen, together with a smalllquantity of C3 and heavier hydrocarbons and absorber lean and/ or spongeoil, is passed to hydrogenation facilities 3l) and a caustic Wash tower32 in order to remove a major portion of the acetylene present andsulphur compounds, organic acids and carbon dioxide. The gaspurification or caustic wash treatment in this stage is well known. Inthe hydrogenation facility, acetylene is hydrogenated to ethylene andother higher molecular weight compounds than ethylene. The hydrogenationmay be effected in the `presence of a suitable catalyst such as nickeloxide.

After washing, the gases are again compressed at 34, this time to apressure in the range of 500-600 p.s.i.g., and are initially cooled inheat exchanger 36 to a temperature of about 60 F. After passing througha separator drum 38, thel moisture content is reduced to a minimum levelin a drier 40, and the gases are subjected to cooling ina multi-stageheat exchanger 42, wherein they are cooled to a temperature of about 80F. The compressed and cooled stream is then fed to a demethanizer 44where it is fractionated under pressure to remove the methane and lowerboiling constituents, such as hydrogen, as a gaseous fraction.

In the demethanizing stage, the overhead product is first passed to areflux condenser not shown where it is partially liquiiied anddischarged into a separator also not shown. In the condenser, arefrigerated medium is used to impart a low temperature to the overheadfraction sufficient to maintain the unliquitied portion of the overheadas substantially a hydrogen and methane fraction, having only a smallamount of ethylene and heavier constituents. The liquid portion of thedemethanizer overhead product is separated from the uncondensed orunliquied portion in the separator, and is discharged into the top ofthe demethanizer tower as redux.

' During cooling the demethanized feed gas, methanol is intermittentlyinjected into the gas feed in line 46 to prevent freezing of the lineyand downstream equipment. In this respect, the dried gas, beingsubstantially free of water, still has a moisture content suicient tocause diniculty. Generally, the methanol is injected when a pressureincrease in the line, indicating plugging of the line, is detected.

The top of the demethanizer tower is operated at a low temperature, somethanol is also injected into the tower in line 48 to prevent freezing.

The liquid bottoms portion is passed by a line 56 leading from thebottom of the demethanizer tower through a pressure reducing valve 58where it may be partially vaporized, toa scrubber column 60 where theethyleneethane gaseous fraction flows upwardly through trays or packingin the column. The column is provided with a vaporizer 62 having aheating coil 64 adapted to vaporize any C2 and lower weight compounds inliquid form, the heavier than ethane hydrocarbons condensing at thebottom of the Vaporizer. The scrubber column in this instance is aminimum tray column operated at a temperature of about 30 F. under apressure of about 335 p.s.i.g.

The vaporized scrubbed gases pass from the top of the column by a line68 to a heat exchange unit 70 where the gas temperature is furtherincreased to about 390 F. and to a second stage hydrogenation facility72. In the scrubber column, the vaporized mixture is passed in countercurrent contact with a reflux liquid withdrawn from the ethylene-ethanesplitter, designated by the numeral 74.

The second stage hydrogenation facility contains a cobalt molybdatecatalyst and under conditions known in the art converts acetyleneremaining in the gaseous stream to ethylene, reducing the acetylenecontent to less than p.p.m. However, as indicated above, in conventionalsystems where the demethanizer bottoms are passed directly to the secondstage hydrogenation facility, the methanol in the gas stream, althoughintermittently injected, is suicient to cause poisoning of thecobalt-molybdate catalyst'.

Following the second stage hydrogenation, the gaseous stream is passedbya line 76 to the ethylene-ethane splitter or fractionation tower 74wherein it is subjected to fractionation under suicient pressure toseparate the ethylene from the ethane and other heavier components. Inthis instance, the splitter is a multiple tray column operated 'at asuperatmospheric pressure which is sufficiently less than the pressurein the demethanizer to overcome the pressure drop through the scrubbertower and hydrogenation Y facilities.

In the splitter column 74, an ethylene fraction is discharged frorn thetop of the column through a line -78 to a condenser 80 where thefraction is partially condensed and passed to a separator 82. Thegaseous fraction being substantially pure .ethylene is withdrawn fromthe separator in line S4 and the reflux liquid fraction is returned tothe tower by reflux pumps 86. The towerA is provided with a reboiler 88in which a heating medium'partially vaporizes the liquid bottoms portionin the bottom of the tower, the residual fraction remaining afterreboiling, principally ethane, being removed from lthe bottom 0f thetower in line 90.

In accordance with the invention, part of the ethyleneethane, liquid,reflux mixture being returned to the tower by pump 86 is withdrawnthrough line 92 and returned as liquid refluxto the vaporizer andscrubber column. It is introduced into the scrubber column labove therst tray, or above the packing, and passes in counter-current contactwith the vapor stream serving to condense substantially all of themethanol and hydrocarbons heavier than ethane from the stream whileallowing substantially all of the ethylene-ethane mixture to ow as vaporto the acetylene hydrogenator. The height of the contact area in thescrubber column and the flow rate of the C2rich liquid stream are theprincipal factors adjusted to achieve a heat transfer whereby the aboveoccurs. Preferably, the height of the contact area is heldl to aminimum. In this instance, the column contains tive trays.

In an example in accordance with the invention, the product stream fromthe bottom of the demethanizer, at a ilow rate of about 19,600 lbs. perhr., will contain about a 4.8 mol percent of methanol and heavierhydrocarbons. In the vaporizer and scrubber, wherein the product streamis contacted with an ethylene lean reflux liquid fed from the splitterat a rate of about 409 lbs. per hr., substantially all of the methanoland hydrocarbons heavier than ethane will be withdrawn as a bottomsmixture containing only a small part of the ethylene and ethane. to thehydrogenator on the other hand will be substantially free of methanol,and will contain less than a 2.5 mol percent of hydrocarbons heavierthan ethane, with a substantially complete elimination of C5 and C6hydrocarbons. Following hydrogenation, trace amounts of acetylene willVbe substantially removed, and after a long period of running,practically no acetylene leakage will occur, indicating improvedcatalyst life. Y

In the ethylene-ethane splitter, .the bottoms mixture will be maintainedat the temperature desired for optimum heat recovery To avoid the use ofa heat exchanger or vaporizer for the scrubber column separate lfrom thereboiler in the demethanizer tower, the relboiler for the demethanizertower may also be used as a vaporizer for the scrubber column.Referr-ing to FIG. 2, the bulk `of the demethanizer bottoms material is-fed by a liue 94 into a reboiler 96 heated by Ia coil 98. While iamajor portion of the vapor-ized stream is returned to the demethanizertower in `line i100, a small portion of the stream is -passed in linei101 to the scrubber column 1102 where it is ysuJbjected tocounter-current contact with reflux withdrawn from the ethylene-ethanesplitter in the manner described with reference -to FIG. 1. Preferably,the scrubber columnin this example Iis operated 'at a pressure equal tothe bottom pressure in the demethanizer tower.

In this embodiment, as illustrated in FIG. 3, as an alternative, it maybe found more convenient .to physically associate the heating coil andlreboiler ywith the scrubber column (103), and to .withdraw a Imajorportion of the vapor from the top of the reboiler inV line 104 andreturn it to the demethanizer. With this arrangement, the struct-ure ofthe scrubber column and vaporizin-g unit would resemble that illustratedin FIG. l, except that the Vaporizing lunit 'would be a larger duty unitto` serve both las a reboiler yand a lvaporizer for the scrubber.

In a third embodiment, FIG. 4, a main portion of the Thefeeddemethanizer bottoms material is withdrawn from a lower part of thedemethanizer tower in line 108 as a side stream vapor and introducedinto the bottom of the scrubber column 110. The vapor -side streamcontains substantially all of the ethylene and ethane present in thedemethanizer, and a small bottom-s liquid stream is withdrawn from thetower .in line y2h12 containing the bulk of the methanol and heavierhydrocarbons. The remaining portion of methanol and heavier hydrocarbonsis substantially removed from the vapor side stream in the mannerdescribed with reference to FIG. l.

Since .the material in the bottom of the demethanizer tower is largelyheavier hydrocarbons, the bottoms temperature may be somewhat higher.Accordingly, the demethanizer would be operated at a lower pressure andlower refrigeration temperature, but only a single heat exchange unit,reboiler ldd, would be required.

Many modifications will .be apparent to those skilled in the art. Forinstance, the iiow sheet arrangement was provided `only by way of`example to establish the relationship of the vaporizing and scrubbingsteps to the remainder of the recovery systems. The invention readilymay be used in other recovery systems. Also, as indicated, the refluxfor the scrubbing column may be withdrawn Ifrom any point on theethylene-ethane splitter. Although the invention has been described withrespect to the use of methanol to prevent freezing, it may also beapplicable with the use of other anti-icing agents, for instanceethanol. Other arrangements are iwithin the spirit and scope of theinvention.

Accordingly, the invention is to be limited in scope only as defined inthe yfollowing claims.

W'hat is claimed is:

1. A process for the recovery od ethylene from la mix- -ture of lightgases including methane, 'hydrogen and acetylene, comprising the stepsof compressing and cooling the mixture and subjecting it todemethanization to separate the hydrogen and methane as .a `gaseousfraction, this step including adding an anti-icing agent to the gas feedto prevent reezing; vaporizing the demethanizer liquid bottoms material;hydrogenating the vaporized bottoms in a hydrogenation facility toconvert traces of the acetylene to ethylene; and fractionating thehydrogenated stream in yan -ethylene-ethane splitter to obtainsubstantially pure ethylene; the improvement comprising subjecting theyaporized, demethanizer bottoms stream prior to hydrogenation toscrubbing in countercurrent contact with a Ca-rich liquid ywithdrawn:from the ethylene-ethane splitter to condense the anti-icing agent andhydrocarbons heavier thanl ethane yfrom the stream.

2. A process according to claim l wherein the CZ-rich liquid is aportion of the condensed reflux from the ethylene-ethane splitter.

3. A process according to claim 1 wherein the demethanizer bottomsstream prior to hydrogenation is fed into a vaporizer scrubber columnhaving a contact area, the Cg-rich liquid stream being introduced abovethe contact area, the demethanizer bottoms stream being introduced belowthe contact area but above the vaporizer.

4. A process according to claim 3 wherein the height of the contact areain the scrubber column and ilow rate of the Cg-rich liquid stream varesuiicient to effect a heat transfer lfrom the -vaporized anti-icingagent and heavier than ethane hydrocarbons in 4the demethanizer bottomsto the liquid stream whereby the former are condensed, the area ofcont-act being the minimum required.

5. A process according to claim 1 including an absorption zone prior todemethanization for obtaining an ethane and lighter fraction wherein thedemethanizer `feed mixture includes amounts of absorber oil used in theabsorption zone, substantially complete elimination of the oil from theethylene-ethane mixture by the scrubbing liquid being eiected.

6. A process for the 1recovery of ethylene `from a mixture of lightgases including methane, hydrogen and acetylene, comprising the steps ofcompressing and cooling the mixture, iand introducing methanol into thelfeed stream t-o prevent freezing; feeding the mixture into ademethanizer to'wer having a reboiler and subjecting it todemethanization under reflux conditions, this step including addingmethanol to the demethanizcr tower to prevent freezing; vapor-izing thedemethanizer bottoms material; hydrogenating the vaporized bottomsmaterial in a hydrogenlation facility to convert traces of the acetylene-to ethylene; and -fractionating the hydrogenated stream in anethylene-ethane split-ter to obtain substantially pure ethylene; theimprovement comprising subjecting the vaporized bottoms material prior.to hydrogenation to scrubbing in counter-current contact with a C2-richliquid withdrawn from the ethylene-ethane splitter to condense themethanol and 'heavier hydrocarbons from the stream.

7'. A process according to claim 6 wherein a single vaporizer unitserves as the demethanizer reboiler and as means to vaporize theethylene-ethane mixture prior to hydrogenation` 8. A process for .therecovery of ethylene from a mixture of light gases including methane,hydrogen 'and acetylene, 'comprising the steps of compressing andcooling the mixture and subjecting it to demethanization to separate'the hydro-gen and methane as a gaseous fraction, -this step includingadding methanol to the gas feed to prevent freezing; vaporizing thedemethanizer bottoms material; returning a portion of the vaporizedbottoms to the dem-ethanizer tower and hydrogenating the remainingportion in a hydronation ifacility to convert ltraces of acetylene toethylene; and fractionating the hydrogenated stream in anethylene-ethane splitter to obtain substantially pure ethylene; theimprovement comprising subjecting the portion of the demethanizerbottoms material to be hydrogenated prior to hydrogenation to scrubbingby counter-current contact with Cz-ric-h liquid withdrawn from theethylene-cth ane `splitter to condense the methanol and hydrocarbonsheavier than cth-ane from Ithe stream.

9. A process for the recovery of ethylene from 'a mixture lof lig-htgases including methane, hydrogen and acetylene and requiring theremoval of `all but about 5 ppm. of lthe acetylene, which includes thesteps 'of deethanizing the mixture in an absorber-deethanizer containingan yabsorber oil; passing the hydrogen, ethane .and lighter hydrocarbonmixture containing la small quantity of C3 hydrocarbons and .absorberoil to iirst stage hydrogenation and caustic treating facilities toremove sulphur compounds and a major portion of the acetylene;compressing, refr-igerating and drying the gaseous stream, and addingmethanol to the feed stream to prevent freezing; demethanizing themixture under reilux conditions to remove methane .and hydrogen;vaporizing the demethanizer bottoms material and hydrogenating a portionof the same in a second stage hydrogenation facility to remove the lasttraces off acetylene; and processing the hydrogenated material in anethylene-ethane splitter to recover Substantially pure ethylene as anoverhead product from the splitter; the improvement comprisingsubjecting the portion of the vaporized dernethanizer bottoms to be4hydrogenated prior to hydrogenation to a scrubbing step with a refluxCZ-rich liquid stream withdrawn from any Itray of the ethylene-ethanesplitter, the scrubbing step serving to condense substantially iall ofthe methanol and hydrocarbons heavier than ethane lfrom the gas stream.

lll. A process Ifor the recovery of ethylene vfrom a mixture of lightgases including methane, hydrogen and acet ylene 'comprising the stepsof compressing and cooling the mixture, and introducing methanol intothe feed stream to prevent freezing; feeding the mixture into ademethaniner tower having a reboiler and subjecting it todemethanization under reilux conditions; withdrawing a vapor side streammixture from a lower part of the de- References Cited by the Examiner YUNITED STATES PATENTS 1 2,804,488V s/57 Cobb 26o-fewV 5. 2,938,934 5/603Williams 26e-677 3,003,008 10/61 'Fleming et al.V 260-677 ALPHONSO D.SULLIVAN, Primary Examiner.

JAMES S. BAILEY, Examiner.

1. A PROCESS FOR THE RECOVERY OF ETHYIENE FROM A MIXTURE OF LIGHT GASESINCLUDING METHANE, HYDROGEN AND ACETYLENE, COMPRISING THE STEPS OFCOMPRESSING AND COOLING THE MIXTURE AND SUBJECTING IT TO DEMETHANIZATIONTO SEPARATE THE HYDROGEN AND METHANE AS A GASEOUS FRACTION, THIS STEPINCLUDING ADDING AN ANTI-ICING AGENT TO THE GAS FEED TO PREVENTFREEZING; VAPORIZING THE DEMETHANIZER LIQUID BOTTOMS MATERIAL;HYDROGENATING THE VAPORIZED BOTTOMS IN A HYDROGENATION FACILITY TOCONVERT TRACES OF THE ACETYLENE TO ETHYLENE; AND FRACTIONATING THEHYDROGENATED STREAM IN AN ETHYLENE-ETHANE SPLITTER TO OBTAINSUBSTANTIALLY PURE ETHYLENE; THE IMPROVEMENT COMPRISING SUBJECTING THEVAPORIZED, DEMETHANIZER BOTTOMS STREAM PRIOR TO HYDROGENATION TOSCRUBBING IN COUNTERCURRENT CONTACT WITH A C2-RICH LIQUID WITHDRAWN FROMTHE ETHYLENE-ETHANE SPLITTER TO CONDENSE THE ANTI-ICING AGENT ANDHYDROCARBONS HEAVIER THAN ETHANE FROM THE STREAM.